I have the canon 1.4xlll and it is a great tc. As others have mentioned, it can depend on your future plans for lens purchases. I also have the sigma extenders, but use them only on my sigma 50-500os. What can you say about the 50-500? It is ok, but often i use a good bit of sharpening in ps6, especially using the tc(1.4-the 2.0 is not that sharp & i rarely use it). Interesting that i just bought the kenko 1.4 for my 70-300L because some posters here on CR said that it is a fairly decent tc, but have not had time to try it out.